Coated cellulosic pellicles and methods of producing the same



United States Patent @firce 2,721,150 Patented Oct. 18, 1955 COATEDCELLULOSIC PELLICLES AND RETHODS OF PRODUCIN-Z? THE SAME ApplicationDecember 12, 1952, Serial No. 325,713

10 Claims. (Cl. 117-145) N Drawing.

This invention relates to improved cellulosic pellicles, and moreparticularly to cellulosic pellicles comprising an anti-sticking agentand having improved surface slip characteristics. The invention is partiularly applicable to regenerated cellulose sheeting or film which can berolled upon itself or stacked without caliing or sticking.

Transparent regenerated cellulose sheeting is produced commercially byregeneration of cellulose rorn, for example, viscose, followed bywashing, bleaching and otherwise purifying the sheets, impregnating thewet or gel sheets with an aqueous solution of a suitable plasticizer andfinally drying the piasticized sheets over a plurality of heated rolls.In order to preserve the toughness and plasticity of the sheets, it isnecessary to maintain a predetermined moisture content therein. The bestresults are obtained when the sheet contains moisture in equilibriumwith the surrounding atmosphere having a relative humidity ofapproximately 35 per cent, i. e., approximately per cent to 8 per centmoisture, and during manufacture of the sheets such a moisture contentis obtained during drying of the sheets by careful control and constantsupervision and analysis. The plain sheets as manufactured, that is,sheets which have not been coated with a moistureproofing composition,are extremely sensitive to humidity changes and the amount of moistureabsorbed thereby varies over a wide range in response to variations inthe humidity of the surrounding atmosphere. Glycerol or otherhygroscopic plasticizing or softening agent is incorporated in the gelsheet to insure absorption of moisture adequate for plasticity at lowrelative humidity of the atmosphere to which the sheets are exposed.However, because of the inherent hydrophilic character of theregenerated cellulose and the presence of the glycerol or otherhygroscopic agent therein the sheets absorb more moisture than isrequired for workable plasticity when they are subjected to highrelative humidities with the result that the surfaces of the sheets tendtoward tackiness. Surface tackiness or the tendency of the surfaces tocohere when the sheets are assembled in stacks or rolled upon themselvesincreases with increase in moisture content above the maximum needed forretention of workable plasticity and results in caking or blocking ofthe stacked or rolled sheets so that they cannot be readily separatedfor use in wrapping or packaging.

It has been suggested heretofore to eliminate this difficulty bytreating the surface of the film with an antisticking or slip agent.Slip as used herein refers to the amount of friction, or lack of it,between adjacent surfaces of superimposed regenerated cellulose films orsheets and is designated quantitatively by the coefficient of frictionof two surfaces of film when subjected to a load of approximately onelb. per sq. inch and acted on by a force at right angles to the appliedload. Various agents of difierent types have been proposed for use asslip agents, but most of the materials previously suggested have had oneor more defects which detract from their usefulness on a practicalcommercial scale. They may not impart the desired slip to the sheets, asis the case with soluble and insoluble silicates and albuminoussubstances, they may be effective only in relatively large amounts sothat use thereof increases the manufacturing cost of the sheet toogreatly, or they may fail to give good slip at the higher temperaturesto which the sheet is subjected when it is exposed to heat developed byfriction in an automatic wrapping machine. Heretofore resinous materialsgenerally have not been found satisfactory as anti-sticking agents. Ingeneral, resins greatly reduce the cohesion-resistant property of thesheets and also tend to impair the capacity of the sheets to be gluedwith aqueous adhesives. It has been found, for instance, thatmelamineformaldehyde resin, which is of the thermosetting type, does notprevent caking of regenerated cellulose sheets treated with it even whenthe resin is used in the form of a colloidal solution or dispersion ofthe precondensate.

it is an object of this invention to provide cellulosic sheets and filmshaving good slip characteristics over a wide range of relative humidityand temperature conditions. Another object of the invention is toprovide new slip or anti-sticking agents for non-fibrous cellulosicsheets and films. A further object is to provide cellulosic sheets andfilms having superior slip characteristics and comprising a resinousanti-sticking agent, which sheets can be glued by means of aqueousadhesives if desired.

1 have discovered that solid finely divided copolymers of vinylidenechloride and vinyl chloride containing a preponderant proportion ofvinylidene chloride in the molecule are very effective anti-stickingagents for cellulosic films and sheets and can be used in comparativelysmall amounts to prevent surface tackiness of the films and sheetsthroughout the range of temperatures and relative humidities normallyencountered in handling, processing and using the sheets. The aboveobjects, and other objects which will clearly appear hereinafter, areaccomplished, therefore, by the present invention in accordance withwhich the particulate copolymer of vinylidene chloride and vinylchloride is deposited in or on the cellulosic sheet from an aqueousdispersion or emulsion of the copolymer. Although the invention isparticularly applicable to sheets or films of regenerated celluloseproduced by the cuprammonium or viscose processes, it is also applicablegenerally to other cellulosic pellicles which are smooth, non-fibrousand substantially non-porous, especially water-sensitive cellulosepellicles such as those made from solutions of cellulose in organic orinorganic solvents, denitrated cellulose nitrate, waterinsolublecellulose others including water-insoluble alkalisoluble hydroxyalkylcellulose and carboxy alkyl cellulose, mixed ethers of that class,cellulose ether Xanthat'es, cellulose thiourethanes and cellulose Xanthofatty acids. The copolymer of vinylidene chloride and vinyl chloride maybe produced by heating the monomers at temperatures between about 50 C.and about 150 C. under appropriate conditions of polymerization, i. e.,in the presence of suitable diluents and such catalysts as are known topromote the polymerization of unsaturated compounds, using the monomersin any suitable relative proportions. The copolymers should contain, byweight in the polymer molecule, from 60 per cent to 95 per cent ofvinylideue chloride and, conversely, from 40 per cent to 5 per cent ofvinyl chloride. Preferably the copolymer is a copolymer of per cent to85 per cent vinylidene chloride and 15 per cent to 25 per cent vinylchloride, and most desirably it is a copolymer of about per centvinylidene chloride and about 20 per cent vinyl chloride. Aqueousemulsions or latices comprising the copolymers are availablecommercially, and such prepared commercial .productsmay .be used .inpracticing the .invention either as such or after suitable dilution withwater. For example, a commercially available latex or emulsion of saran(a vinylidene chloride-vinyl chloride copolymer marketed by Dow ChemicalCompany) may be used. Or a latex may be prepared -'by any of thewell-known methods used to obtain dispersions or latices, such asspecific emulsifying agents, stabilizing agents, or other substanceswhich produce stable dispersions. Suitable emulsifying agents includesodium lauryl sulfate, sodium dioctyl sulfate, alkaryl acids, sulfonicacids and their salt's, and the like. The emulsion may also contain aplasticizing agent for the copolymer which may be, for example, oftheplasticizing resin type, i. e., a soft resin such as a polymerizedunsaturated hydrocarbon, for instance, polymerized isobutylene commonlysold under the tradename Vistanex, a soft alkyd resin, or an organicplasticizer, such as dibutyl sebacate, dibutyl phthalate, and the likeor mixtures of those plasticizers. Protective colloids may also be addedto the emulsion of the copolymer, and as suitable protective colloidsthere may be mentioned casein, glue, a water-soluble celluloseeth'ersuch as methyl cellulose, or a gum such as arabic, karaya ortragacanth.

The emulsion of the finely divided thermoplastic vinylidenechloride-vinyl chloride copolymer may be applied to a finishedcellulosic sheet, after initial drying thereof, provided that the sheetis re-wet before the emulsion is applied to :it. Also, the particulatecopolymer may be dispersed in a moisture-proofing composition if such isapplied to the finished sheet. However, the application ofmoisture-proofing or water-proofing compositions to the sheetsinvariably results in products Which cannot be glued with commercialaqueous adhesives, and it is frequently desirable to avoid theme ofmoisture-proofing compositions and to use the plain regeneratedcellulose sheet as manufactured for purposes which involve gluing bymeans of aqueous adhesives and for other purposes, for example :in theproduction of envelopes having a cellophane window. Although theanti-sticking agent of the invention may be applied conjointly withmoistureproofing compositions or subsequently to the application of suchcompositions, the invention is particularly concerned with the treatmentof plain cellulosic sheets and generally speaking the best results areobtained by applyi-ng the particulate solid copolymer to the sheet inthe gel state; that is, after the sheet has been purified in the courseof its manufacture but prior to any drying thereof. The copolymer ofvinylidene chloride and vinyl chloride may be applied to the cellulosicsheet during impregnation of the latter with the hygroscopicplasticizing agent by adding the aqueous latex or dispersion of thecopolymer to the plasticizer bath on the casting machine in sufiicientquantity to produce a concentration in the bath of 0.1 per cent to 2.0per cent solids based on the water content of the bath. The gelcellulosic film or sheet is then treated by passage through the bath,the excess antisticking agent as well as the excess softener solutionbeing removed by means of squeeze rolls, scraper rods, wiper blades, orthe like before it enters the drier in which it is dried in the usualmanner. The treated cellulosic sheet does not adhere to the drier rollsand the shrinkage which normally takes place during initial drying of are-' generated cellulose film is not affected. The amount of copolymerwhich is applied to the gel sheet is controlled by adjusting theconcentration of the copolymer in the plasticizing bath or by varyingthe amount of excess removed. The copolymer is elfective to completelyeliminate any tendency of the sheets to stick or block in smallquantities, and a concentration of the copolymer latex in the bathsufiicient to give a solids concentration in the bath of between 0.1 percent and 0.5 per cent is generally sufiicient to prevent sticking of thedried sheets.

The cellulosic pellicle will usually be transparent and colorless, butit is within the scope of the invention to treat colored or otherwisedecoratedpellicles. The presence of the copolymer in the pellicle doesnot interfere with subsequent treatments including laminating, coating,gluing, printing, or the like, and the film may be provided with asurface coating, for example a moistureproofing coating without harmfuleffects due to the presence of the particulate copolymer therein.

The softening agent applied conjointly with the new anti-sticking agentwill usually be glycerol, but the dispersion of the copolymer iscompatible with aqueous solutions of other non-volatile cellulosesofteners, and such other softeners, including diethylene glycol,triethylene glycol, invert cane sugar, glucose, sorbitol,'etc., ormixtures thereof, may be used if desired. The particle size of thewater-insoluble resinous copolymer of vinylidene chloride and vinylchloride is preferably within the range of from about 0.1 to about 8microns in its greatest linear dimension, and most desirablythe'particle size is within the range of 0.5 to 2.5 microns.

The following example, in which the quantities given are in parts byweight unless otherwise specified, illustrates a practical applicationof the invention in the production of transparent, highly softened,non-sticking cellulosic pellicles. In the example the gel sheet treatedcomprises regenerated cellulose, but it will be understood that the useof other cellulosic sheets is contemplated.

Example Different pellicles of gel regenerated cellulose were passedthrough softening baths containing, in addition to glycerol asplasticizer, an aqueous emulsion of a particulate copolymer of by weightabout per cent vinylidene chloride and about 20 .per cent of vinylchloride (or the product marketed commercially by the Dow ChemicalCompany under the trade name Dow Latex 744). The compositions of thebaths are given in the following table.

0 1 s iii opo ymer o s Water Glycerol Emulsion (COBOL ymer) On leavingthe baths the pellicles were passed through squeeze rolls to removeexcess solution and then dried to a moisture content of about 5 to 8 percent by passing them over drier rolls. The sheets were stacked andstored under a pressure of about 1 lb. per sq; inch in an oven at F. for16 hours without exhibiting any tendency to stick or cake.

In contrast, when a gel regenerated cellulose sheet was passed through abath containing by weight 1093.5 parts of water, 5 3 parts of glycerol,and a sufiicient quantity of the product obtained by colloidallydispersing a melamine-formaldehyde resin precondensate in aqueous lacticacid to give a bath containing by weight 23.5 parts of themelamine-formaldehyde resin precondensate and 5 parts by weight oflactic acid (lactic acid concentration in the bath in percent by weight0.425) the sheets, after drying thereof, during which the resin wasbrought to fully condensed, thermoset condition, exhibited a definitetendency to cake when stacked and stored under the same conditions asthe sheets which were treated in the softener baths containing thesoftener and the thermoplastic particulate vinylidene chloride-vinylchloride copolymer.

Although the melamine-formaldehyde resin does not, of itself, preventsticking of the cellulosic sheets, it may be used in conjunction withthe particulate vinylidene chloride-vinyl chloride copolymer forimparting characteristics other than surface slip to the sheets, Withoutdetracting from the effectiveness of the copolymer as an anti-stickingagent. For example, if it is desired to after-treat the sheet comprisingthe particulate vinylidene chloride-vinyl chloride copolymer with amoistureproofing agent or composition, it may be desirable toincorporate the melamine-formaldehyde resin with the gel cellulosicsheet before, simultaneously with, or after incorporation of theparticulate vinylidene chloride-vinyl chloride copolymer, since thosemelamine-formaldehyde resins, the precondensates of which form colloidaldispersions in aqueous solutions of organic acids, are anchoring agentsfor compositions comprising hydrophobic moisture-proofing materials andserve to bond such materials firmly to the hydrophilic cellulosic basesheet. Sheets having both the particulate vinylidene chloridevinylchloride copolymer and extremely small particles of amelamine-formaldehyde resin intimately associated therewith have beenobtained and, on testing for adhesion when rolled and stacked, have beenfound to have slip characteristics comparable to those of similar sheetshaving the vinylidene chloride-vinyl chloride copolymer onlyincorporated therewith. As an example, a regen erated cellulose sheet ingel condition was passed through an aqueous bath containing both theparticulate vinylidene chloride-vinyl chloride copolymer and amelamineformaldehyde resin precondensate. The melamineformaldehyde resinwas introduced into the bath in the form of an aqueous lactic acidcolloidal solution containing between 0.5 per cent and 20 per cent byweight of a partially polymerized melamine-formaldehyde condensationproduct in a state of polymerization less than that characterizing agel, prepared in accordance with U. S. 2,394,009 to John D. Pollard,issued February 5, 1946, the solution having been aged after itspreparation, as set forth in the Pollard patent or in Wohnsiedler andThomas, U. S. 2,345,543, issued March 28, 1944. The bath contained byweight 1083.5 parts of water, 53 parts of glycerol, 23.5 parts of thecolloidal aqueous lactic acid solution of the melamine-formaldehyderesin (0.425 per cent lactic acid) and parts of Dow latex 744 (per centsolids, 0.425). Excess bath was removed from the sheets, which were thendried. During the drying the melamine-formaldehyde partial condensatewas converted to fully condensed, thermoset condition. These sheetsexhibited no tendency to stick or block when stacked under a pressure of1 lb. per sq. inch in an oven at 115 F. for 16 hours.

Since changes and modifications can be made in the specific details asset forth above without departing from the nature and spirit of theinvention, it is to be understood that the invention is not to belimited except as set forth in the appended claims.

I claim:

1. An article of manufacture comprising a non-fibrous cellulosicpellicle carrying on its surface discrete, finelydivided particles of acopolymer of from 60 per cent to 95 per cent of vinylidene chloride andfrom 5 per cent to 40 per cent of vinyl chloride.

2. An article of manufacture according to claim 1 characterized in thatthe pellicle comprises regenerated cellulose.

3. An article of manufacture according to claim 1 characterized in thatthe particulate copolymer is a copolymer of about 75 per cent to 85 percent of vinylidene chloride and about 15 per cent to 25 per cent ofvinyl chloride.

4. An article of manufacture according to claim 1 characterized in thatthe particulate copolymer is a copolymer of about 80 per cent ofvinylidene chloride and about 20 per cent of vinyl chloride.

5. As a new article of manufacture, a regenerated cellulose pelliclehaving incorporated therein a watersoluble polyhydroxy alcoholplasticizer and carrying on its surface discrete, finely-dividedparticles of a copolymer of per cent to 95 per cent vinylidene chlorideand 5 per cent to 40 per cent of vinyl chloride.

6. An article of manufacture according to claim 5 characterized in thatthe water-soluble plasticizer is glycerol.

7. In a method of making a non-sticking, non-fibrous cellulosicpellicle, the steps which comprise treating the pellicle with an aqueousdispersion comprising 0.1 to 2% of a finely-divided particulatecopolymer of 60 per cent to 95 per cent vinylidene chloride and 5 percent to 40 per cent of vinyl chloride, and drying the pellicle.

8. In a method of making a non-sticking, non-fibrous cellulosicpellicle, the steps which comprise treating the pellicle in the courseof its production and prior to initial drying thereof with an aqueousdispersion of 0.1 to 2% of a finely-divided particulate copolymer of 60per cent to 95 per cent vinylidene chloride and 5 per cent to 40 percent of vinyl chloride, and drying the pellicle.

9. In a method of making a softened, plastic, nonsticking, non-fibrousregenerated cellulose pellicle, the steps which comprise treating thepellicle, after washing thereof in the course of its production andprior to initial drying thereof, with an aqueous medium containing awater-soluble polyhydroxy alcohol plasticizer for regenerated celluloseand 0.1 to 2% of a dispersed finelydivided particulate copolymer of percent to 85 per cent vinylidene chloride and 15 per cent to 25 per centof vinyl chloride, removing excess treating medium, and drying thepellicle.

10. The method according to claim 9 characterized in that theplasticizer is glycerol and the copolymer is a copolymer of about percent vinylidene chloride and about 20 per cent vinyl chloride.

References Cited in the file of this patent UNITED STATES PATENTS2,334,236 Arnold et a1. Nov. 16, 1943 2,417,014 Pollard Mar. 4, 19472,570,478 Pitzl Oct. 9, 1951 2,618,575 Oswin Nov. 18, 1952

1. AN ARTICLE OF MANUFACTURE COMPRISING A NON-FIBROUS CELLULOSICPELLICLE CARRYING ON ITS SURFACE DISCRETE, FINELYDIVIDED PARTICLES OF ACOPOLYMER OF FROM 60 PER CENT TO 95 PER CENT OF VINYLIDENE CHLORIDE ANDFROM 5 PER CENT TO 40 PER CENT OF VINYL CHLORIDE.